1. Field of the Invention
The present invention relates to a method for fabricating a press forming die, a method for fabricating a press forming punch, and a press forming die assembly, which die, punch and assembly are suitable for press forming a blank material into a product panel.
2. Description of the Related Art
Many of component parts such as a fuel tank of a two-wheeled vehicle are fabricated chiefly by press forming with the use of a press forming die assembly, which is generally formed of alloy steel shaped by precision work. The press forming die assembly of this type is advantageous in that it has great durability. As a result, even if the cost of the press forming die assembly is extremely high, it is possible to collect the cost by mass production.
Since, however, model changes in modern two-wheeled vehicles tend to be carried out in a relatively short cycle time and the mass production is inclined to a smaller-volume production with many kinds. If the press forming die assembly made of alloy steel is employed in smaller-volume production with many kinds, it is difficult to collect the die costs and to restrain the cost hike of the products.
To solve this problem, press forming die assemblies made of plastic (hereinafter referred to as plastic type press forming die assemblies) are widely used for the smaller-volume production with the many kinds. Since the plastic type press forming die assembly can be simply fabricated, the cost of the plastic die assembly is lower than that the press forming die assembly of the alloy steel type. As a result, even when the model changes of the two-wheeled vehicles are frequently made in the short cycle time, it is easy to pay off and the cost up of the products can be restrained. However, since the plastic type die assembly has a die surface softer than the alloy type die assembly, the die surface of the plastic type die assembly is easy to wear. When this occurs, it is difficult to maintain the accuracy of the press formed products. Accordingly, in order to maintain the press formed products at a high accuracy, replacements of the plastic type die assemblies are frequently required.
For this reason, the old plastic type die assembly should be replaced with new one within a relatively short working cycle, resulting in a difficult collection of the die costs.
Further, it has been proposed to employ zinc alloy steel (for example, ZAS) for press forming die assemblies to reduce the die costs. Since a principal ingredient of ZAS is zinc, it melts at a low temperature of, for example, about 300xc2x0 C. Consequently, it is possible to reduce the die costs of the die assembly of the ZAS type.
Since, however, the ZAS type die assembly also has a relatively soft die surface, the die surface of the ZAS type die assembly has a lower wear resistant property than the alloy steel type die assembly. For this reason, the old ZAS type die assembly should also be frequently replaced with a new one in a relatively short cycle time, resulting in a difficult collection of the die costs.
It is therefore desired to provide a new type of press forming die assembly that is enabled to restrain the die costs while having prolonged durability close to that of the alloy steel type die assembly.
It is therefore an object of the present invention to provide a method of fabricating a press forming die, a method of fabricating a press forming punch, and a press forming die assembly, which are low in die costs and have prolonged durability.
According to one aspect of the present invention, there is provided a method of fabricating a press forming die which comprises the steps of preparing a die forming prototype and a punch forming prototype, press forming a first blank material with the die forming prototype and the punch forming prototype to obtain a shaped panel, fitting the shaped panel into the punch forming prototype again, placing a second blank material, a resilient sheet and the die forming prototype over the shaped panel fitted to the punch forming prototype in a sequential order, moving the die forming prototype toward the punch forming prototype relative to one another and stopping the die forming prototype at a position ahead of a bottom dead center of a press forming machine by a given distance for thereby press forming the second blank material with the shaped panel to obtain a die forming panel, and adding a backup material onto a rear surface of the die forming panel to fabricate the press forming die.
In the above fabrication process, a shaped panel is fabricated by press forming with the die forming prototype and the punch forming prototype, with the shaped panel being further press formed with the punch forming prototype into the die forming panel. The die forming panel is utilized as the die surface of the press forming die. Since, in this event, the die surface of the press forming die can be simply fabricated by press forming work, it is unnecessary to shape the die surface of the press forming die by cutting work with the use of an NC machining center or by profiling with a profiling machine. Accordingly, the press forming die can be fabricated in a simple manner, with a resultant decrease in the die costs. Also, the die surface of the press forming die may be formed by a metal surface having a strong hardness such as high tension steel or stainless steel. In addition, during press forming the die forming panel, the die forming panel may be subjected to a suitable hardening treatment. Consequently, the surface hardness of the die forming panel may be easily hardened to a higher level in the above fashion, thereby increasing the durability of the die surface of the press forming die.
In the preferred method discussed above, the fabrication process may further include, after the step of obtaining the die forming panel, a step of mounting the die forming panel onto a frame body, and a step of filling a high strength cement into a space between the frame body and the die forming panel. The high strength cement is hardened and, therefore, the press forming die can have a highly improved durability.
According to another aspect of the present invention, there is provided a method of fabricating a press forming punch which comprises the steps o f preparing a die forming prototype and a punch forming prototype, press forming a first blank material with the die forming prototype and the punch forming prototype to obtain a shaped panel, fitting the shaped panel into the die forming prototype again; placing a second blank material, a resilient sheet and the prototype punch over the shaped panel fitted to the die forming prototype in a sequential order, moving the punch forming prototype toward the die forming prototype relative to one another and stopping the punch forming prototype at a position ahead of a bottom dead center of a press forming machine by a given distance for thereby press forming the second blank material with the shaped panel to obtain a punch forming panel, and adding a backup material onto a rear surface of the punch forming panel to fabricate the press forming punch.
In the above fabrication process, a shaped panel is fabricated by press forming with the die forming prototype and the punch forming prototype, with the shaped panel being further press formed with the die forming prototype into the punch forming panel. The punch forming panel is utilized as the die surface of the press forming punch. Since, in this event, the die surface of the press forming punch can be simply fabricated by press forming work, it is unnecessary to shape the die surface of the press forming punch by cutting work with the use of an NC machining center or by profiling with a profiling machine. Accordingly, the press forming punch can be fabricated in a simple manner, with a resultant decrease in the die costs. Also, the die surface of the press forming punch may be formed by a metal surface having a strong hardness such as high tension steel or stainless steel. In addition, during press forming the punch forming panel, the punch forming panel may be subjected to a suitable hardening treatment. Consequently, the surface hardness of the punch forming panel may be easily hardened to a higher level in the above fashion, thereby increasing the durability of the die surface of the press forming punch.
According to the second aspect of the invention, the fabrication process may further include, after the step of obtaining the press forming panel, a step of mounting the press forming panel onto a frame body, and a step of filling a high strength cement into a space between the frame body and the press forming panel. The high strength cement is hardened and, therefore, the press forming punch has the backup material that is discussed above.
According to a third aspect of the present invention, there is provided a press forming die assembly which comprises a press forming die, and a press forming punch, each of the press forming die and the press forming punch including a panel for the die assembly that is shaped by press forming a blank material, a frame body mounting thereon the panel, and a backup material filled between a space between the frame body and the panel and composed of either one of cement or plastic.
In accordance with the press forming die assembly, since a die forming panel is fabricated by press forming, the die forming panel can be fabricated with a metal having a strong surface hardness such as high tension steel or stainless steel. Consequently, the die forming panel may be utilized as a die surface of the press forming die, thereby increasing the durability of the die surface of the press forming die. Also, since the backup material for the die forming panel is formed of cement or plastic, it is possible to reduce the die costs.